The animation frame index mask for a guardian is found in bits 5-7 of byte 1 of its buffer (see 33024) while it is on-screen, having been copied there from the corresponding entity definition (see 40960). The mask determines the animation frames that the guardian's sprite cycles through. If 'B' is the guardian's base frame (0-7), then its sprite will cycle through animation frames as follows, where '|' denotes a bitwise 'OR' operation (see 37393):

Mask	Frames
000	B (1 frame)
001	B, B|1 (1 or 2 distinct frames)
010	B, B, B|2, B|2 (1 or 2 distinct frames)
011	B, B|1, B|2, B|3 (1, 2 or 4 distinct frames)
100	B, B, B, B, B|4, B|4, B|4, B|4 (1 or 2 distinct frames)
101	B, B|1, B, B|1, B|4, B|5, B|4, B|5 (1, 2 or 4 distinct frames)
110	B, B, B|2, B|2, B|4, B|4, B|6, B|6 (1, 2 or 4 distinct frames)
111	B, B|1, B|2, B|3, B|4, B|5, B|6, B|7 (1, 2, 4 or 8 distinct frames)

For example, if a guardian's base frame is 2 and its animation frame index mask is 011, then its sprite will cycle through frames 2, 2|1, 2|2 and 2|3 (i.e. frames 2 and 3).

Note that the animation frame masks 100, 101 and 110 are not used by any guardians in the game, and no guardian has a base frame of 7.

A guardian, arrow or rope.

One of the 128 8-byte blocks located at 40960 onwards. Each one defines the location and movement characteristics of a guardian, arrow or rope.

One of the eight pairs of bytes at the end of a room definition (e.g. at 53488). Each one specifies a guardian, arrow or rope present in the room.

An inhabitant of a room that continuously moves left and right (horizontal guardian) or up and down (vertical guardian) in a straight line, or stays still. Contact with a guardian is fatal. Some examples are:

The Security Guard
The Forgotten Abbey
The Bathroom

The graphic data for all the guardians in the game can be found at 43776.

An immobile object drawn in a single cell as part of a room along with the background, walls, floors, ramp and conveyor. Contact with a nasty is fatal. Some examples are:

The Hall
Cold Store
The Wine Cellar